In general, mobile communication devices of this kind (hereinafter, simply called mobile devices) are constituted such that:
1) When the power supply is turned on, the mobile device scans control channels, and selects the control channel having the greatest field strength as the control channel by which it is to transmit and receive control signals to and from a base station (P-CH scan). PA1 2) When a control channel has been selected, the mobile device receives control data transmitted by the base station using the selected control channel so as to be ready for calling operation by the mobile device or receiving an incoming call signal from the base station (standby state). PA1 3) When the mobile device receives an incoming call signal from the base station during the standby state, it responds to this incoming call signal by transmitting an incoming call response signal to the base station. PA1 4) The mobile device receives a communication channel designation signal transmitted by the base station in response to this incoming call response signal, and switches to a communication channel designated by the communication channel designation signal.
Conventional communication systems using mobile devices of this kind are constituted in such a manner that an incoming call signal is transmitted from a base station to a mobile device, and if, for some reason or other, the base station cannot receive the incoming call response signal transmitted by the mobile device in response to this incoming call signal, then the incoming call signal is retransmitted from the base station to the mobile device.
The time interval from the base station transmitting a first incoming call signal until it retransmits the signal, in other words, until it transmits a second incoming call signal because an incoming call response signal from the mobile device cannot be received, is set to approximately 6 seconds, for example.
This interval is determined by taking into account the tolerable waiting time for the party originating the call and the time required for the mobile device to transmit an incoming call response signal after receiving an incoming call signal, and the like. If this time interval is too short, it will not allow sufficient time for dispatch of the incoming call response signal by the mobile device, while if it is too long, the waiting time for the calling party will become long, which is undesirable.
The mobile device is constituted such that if it has transmitted an incoming call response signal to the base station, but does not receive a communication channel designation signal from the base station within a prescribed period of time, then the mobile device regards the transmission of the incoming call response signal as having failed, and it carries out a further P-CH scan, whereby it scans the control channels again and selects the control channel having the strongest electric field as the control channel for transmitting and receiving control signals to and from the base station.
The prescribed time period by which it is judged that no communication channel designation signal has been received from the base station if the prescribed time period has passed after the mobile device has transmitted an incoming call response signal. This prescribed time period used here is set, for example, to approximately 5 seconds.
This time period from transmission of an incoming call response signal by the mobile device until it is judged that the transmission of the incoming call response signal has failed from the fact that no communication channel designation signal has been received, is determined on the basis of the time period required by the base station to transmit a communication channel designation signal after it has received an incoming call response signal. If this time period is too short, it will not allow sufficient time for transmission of the communication channel designation signal by the base station, while if it is too long, the device will wait needlessly for a communication channel designation signal which cannot be received. Especially, in a situation where the mobile device is waiting to receive this communication channel designation signal using a control channel which is being shared with another mobile device, the aforementioned time period cannot be made too long from the viewpoint of efficient use of the radio channels.
Therefore, at the time that the second incoming call signal transmitted by the base station reaches the mobile device, the mobile device may have transferred to carrying out of a P-CH scan, which scans control channels and selects the control channel having the greatest field strength as a control channel for transmitting and receiving control signal to and from the base station, because it considered the transmission of the incoming call response signal to have failed. If this transferring to P-CH scan takes place, a problem may arise in that the mobile device will not be able to receive the second incoming call signal transmitted by the base station.
FIG. 6 gives an illustration, in the form of a sequence chart, of transmission and reception of control signals between a base station and a mobile device in a case where the aforementioned problem occurs.
When the power supply is turned on (Power Supply ON), the mobile device scans the control channels and selects the control channel having the greatest field strength as a control channel to use for transmitting and receiving control signal with a base station (P-CH Scan).
Thereupon, when the mobile device has received control signal from the base station by means of the control channel selected as described above, it assumes a standby state where it awaits a calling operation by the mobile device or an incoming call signal from the base station.
If the mobile device receives a (first) incoming call signal from the base station in this standby state, it assumes a connection control state where a communication channel is established between the mobile device and the base station.
In this connection control operation, firstly, an incoming call response signal is transmitted by the mobile device to the base station.
However, if the base station cannot normally receive the incoming call response signal, then the base station transmits a further (second) incoming call signal to the mobile device, approximately 6 seconds after sending the (first) incoming call signal.
On the other hand, if the mobile device has transmitted an incoming call response signal, but has not received a communication channel designation signal from the base station within approximately 5 seconds, then it regards the transmission of the incoming call response signal to have failed, and carries out a P-CH scan once again by scanning the control channels and selecting the control channel having the greatest field strength as a control channel for transmitting and receiving control signal with the base station.
In the case illustrated in FIG. 6, since the incoming call response signal from the mobile device has not been received normally at the base station, the mobile device transfers to a P-CH scan approximately 5 seconds after sending the incoming call response signal, and the second incoming call signal from the base station reaches the mobile device while it is engaged in the P-CH scan. In this case, the mobile device is not able to receive the (second) incoming call signal from the base station, and therefore cannot respond to the (second) incoming call signal.
Consequently, the mobile device will not be able to respond to the incoming call in spite of the fact that the base station has transmitted a (second) incoming call signal.
In this way, in the conventional mobile device, in cases where the base station transmits a first incoming call signal to the mobile device, and the base station cannot receive an incoming call response signal sent by the mobile device in response to the incoming call signal for a reason of any kind, and the base station therefore transmits a second incoming call signal to the mobile device, a problem may arise in that, at the point in time that this second incoming call signal reaches the mobile device, the mobile device may have transferred to a P-CH scan, namely, a P-CH scan whereby, having considered the transmission of the incoming call response signal to have failed, the mobile device scans the control channels once again and selects the control channel having the greatest field strength as a control channel for transmitting and receiving control signal with the base station. Consequently the mobile device will not be able to receive the second incoming call signal transmitted by the base station.